In the apparatus of German Patent Publication No. 198 26 047, the lower pressure space or working space of the double-acting working cylinder, and the upper pressure space or working space of the working cylinder, are connected to the hydraulic pressure supply device via the control valve embodied as a 2/2-way solenoid valve with spring return. The pressure impingement surface or effective surface of the positioning piston delimiting the upper working space is larger than the pressure impingement surface or effective surface of the positioning piston delimiting the lower working space, so that upon opening of the control valve, a compressive force displacing the positioning piston against the pressure in the lower working space acts on said piston, and the positioning piston opens the gas exchange valve. The upper working space is additionally connected, via a second control valve also embodied as a 2/2-way solenoid valve with spring return, to a return line opening into a fluid reservoir.
To displace the positioning piston in the valve-opening direction, the second control valve is closed and the first control valve opened. As a result of the differing effective surfaces of the positioning piston, the positioning piston is displaced downward and opens the gas exchange valve over a valve stroke that depends on the control valve activation duration. The valve stroke speed depends on the magnitude of the fluid pressure or hydraulic pressure applied by the pressure supply unit. To close the gas exchange valve, the two control valves are switched over so that the upper working space is on the one hand closed off from the pressure supply device and on the other hand connected to the return line. The positioning piston is displaced upward by the pressure present in the lower working space, and closes the gas exchange valve.
To hold the gas exchange valve in the closed position after a complete depressurization of the pressure system resulting from a slight leakage, e.g. when the internal combustion engine is shut off for an extended period or in the event of failure of the pressure supply device, an emergency closure spring is provided which is inserted as a compression spring into the lower working space and is braced against the positioning piston. The emergency closure spring is dimensioned so that in all conditions it overcomes the frictional torques in the gas exchange valve and in the valve positioner, and is capable of moving the positioning piston out of any of its displacement positions into the closed position.